The present invention relates to a method for producing graphite structures which are of light weight and have improved elasticity.
Generally available carbon materials, whether carbonaceous or graphitic, are characterized in that they are of a rigid structure and possess high elasticity (Young's modulus). Light-weight carbon materials, on the other hand, include carbon foams, hollow carbon spheres and expandable graphite.
Carbon foams have been produced either by foaming, curing and calcining polyurethane or phenol resins or by forming and calcining hollow carbon spheres with the aid of a binder. Such foams are found to have a bulk density of the order of about 0.5 g/cm.sup.3, but their graphitized structures have poor flexibility and are thus rigid.
Hollow carbon spheres have been produced by melting and atomizing foamer-containing pitches in the spherical form, which are then calcinated (see Amagi, "Materials", Vol. 16, page 315, 1971). Such spheres are relatively light-weight materials, as expressed in terms of bulk density of 0.1 to 0.3 g/cm.sup.3, but are rigid for lack of flexibility.
Expandable graphite has generally been made by oxidation and heat-treatment of naturally occurring scaly graphite. This graphite is as light in weight as expressed in terms of its coefficient of expansion which may reach a factor of several hundreds, but may be subjected to compression molding, as will be appreciated from the fact that it is usable as the starting material for graphite sheets. Graphite sheets obtainable from such expandable graphite are flexible and possess elasticity to such an extent that they are restorable to their original form after a compression load has been applied thereto and removed therefrom. For this reason, they are said to excel in air-tightness when used as packing materials. However, such sheets are of a densified structure and are not satisfactory whatsoever on the point of recovery.
We have found that graphite structures of light weight and excelling in elasticity are obtainable by treating a carbonaceous material with nitric acid or a nitric-sulfuric acid mixture and graphitizing the thus treated material at a temperature of 2,400.degree. C. or higher, and have proposed a method for making graphite structures on the basis of such findings (Japanese Patent Application No. 285493/1986). Although such graphite structures excel in elasticity, yet they are not necessarily satisfactory in terms of recovery relative to compressibility.
As a result of extensive study made so as to obtain elastic graphite structures having improved recovery relative to compressibility, we have found that elastic graphite structures of markedly improved recovery relative to compressibility can be prepared by using as the graphite structural material a modified carbonaceous material which is obtained by treating a carbonaceous material with nitric acid or a nitric-sulfuric acid mixture, then bringing the thus treated material into contact with a basic aqueous solution to make it soluble, and finally adjusting the pH of the obtained soluble matter, thereby precipitating a carbonaceous component. Thus, we have successfully accomplished the present invention.